1. Field of the Invention
The present invention relates to a method and a device for visual training which are suitable for easing eyestrain of a trainee by displaying targets to his or her eyes and moving the targets in optical axis directions as well as in a convergent direction or a divergent direction (in a horizontal direction of both eyes).
2. Description of the Related Art
In this age of information society, our eyes are often overworked through visual works at near distances such as works on the screens of a television, a personal computer, and the like.
Normally, our eyes see various objects at various distances so that the musculus ciliaris of the eyes repeat relaxation and contraction for appropriate accommodation. However, excessive near-distance works on the screen of a personal computer or the like and keep viewing an object at a same distance cause the eyes not to perform accommodation. As a result, the musculus ciliaris are kept strained for a long time. The long period of strain causes the musculus ciliaris to suffer from fatigue which leads to strain of an eye accommodation system to result in decreased vision. It is said that accommodative spasm occurs if the strained state of the musculus ciliaris further persists. This brings about troubles in visibility and various kinds of stresses.
There have been proposed devices for easing eyestrain by relaxing the tension of the musculus ciliaris of the eye through training so as to eliminate fatigue.
A first example of the proposed devices is simply structured to move a target for the eyes of a trainee from near to far. The trainee's keeping watching the target, following the movement of the target from a close position to a distant position can allow the musculus ciliaris to change from a strained state to a relaxed state. In other words, the movements of the target help the musculus ciliaris be trained. (disclosed in Japanese Unexamined Patent Application Publication No. 6-339501).
However, such a device is disadvantageously large in size to be installed in households or hospitals because the target serving as a tracked object is moved from a very close position to the eyeball of the trainee such as 20 cm from the eye to a far position, for example, about 2 m from the eye.
A second proposed optical device is an optical device which uses an optical lens to reduce a distance of a moving target for the purpose of reducing the size of the device. This device achieves an effect equivalent to that of the first device in which the trainee keeps watching the moving target over a long distance from a position immediately before the eye. Furthermore, this optical device can change a distance in an optical axis direction while moving the target in accordance with a horizontal change of the eyes of the trainee (referred to as convergence or divergence), realizing a situation close to a real working situation of the eyes. (disclosed in Japanese Unexamined Patent Application Publication No. Hei 10-282449).
Herein, the above-mentioned horizontal change of the eyes is explained. As shown in FIG. 8, when a person sees an object with his or her eyes at different distances, the visual axes of both eyes are nearly parallel to each other at seeing from a long distance. On the other hand, at seeing from a shot distance, the visual axes of the eyes goes inward from the parallel state. This is generally called as convergence (represented by a convergence angle θ in FIG. 8). It is apparent that the shorter the distance, the larger the convergence angle θ.
In FIG. 8, the following relationtan θ=P/2L  (1)is established where P is a distance between both eyes, and L is a distance to a target object.
Herein, the power of a lens is generally expressed in a unit of a diopter Dp, and has the following relation:Dp=1/L  (2).
In the second device described above, the visual axes of the eyes are never out of a parallel state by convergence; that is, the visual axes of the eyes never incline outward toward the end toward the end.
In the above-described first and second devises of prior art, the target is repeatedly moved, and the same repetitive movements over a certain period of time are necessary, which bores the trainee to view the target and quit the training in some cases. Therefore, these devices are disadvantageous in that it is difficult to continue the training for a long period of time.
These prior art devices have another problem that training of a short period of time is not enough to ease strained musculus ciliaris of the eyes and eliminate fatigue.